The measurement of the 3D surface microgeometry (also referred to as topography) with high depth resolution has proven to be very difficult and is the focus of active research in the fields of 3D metrology and computer vision. In the field of 3D metrology, a non-contact optical approach is of particular interest driven by customer requirements for greater simplicity and higher speed. Some conventional techniques that can be used to measure surface depth with nanometer to micron-scale depth resolution include white light interferometer, confocal microscope, and focus variation. Conventional instruments that use these techniques generally have, however, a narrow field-of-view, short working distance, and shallow depth-of-field since they use high magnification microscope objectives. These conventional instruments also tend to be very expensive due to their use of sophisticated optical and mechanical components. Moreover, the devices based on interferometry are commonly sensitive to the ambient environment, such as vibration. Also, most of these techniques are designed to take surface topography measurements, and are unable to capture reflectance properties such as diffuse reflection and specular reflection. An example of a white light interferometer can be found in De Groot, P., “Principles of interference microscopy for the measurement of surface topography,” Advances in Optics and Photonics, vol. 7, no. 1, pp. 1-65 (2015), which is hereby incorporated by reference for this example. An example of focus variation device can be found in Matilla, A., et al., Three-dimensional measurements with a novel technique combination of confocal and focus variation with a simultaneous scan,” Proc. SPIE, pp. 98900B-11, (2016), which is hereby incorporated by reference for this example.
In the field of computer vision, some research has been done on 3D surface measurement with somewhat simpler setups. The photometric stereo technique is a method to reconstruct surface normal and depth by capturing multiple images under different lighting directions, but this method assumes the scene to be Lambertian (diffusive). An example of a method to reconstruct surface normal and depth by capturing multiple images under different lighting directions can be found in Basri, D., et al., “Photometric stereo with general, unknown lighting,” International Journal of Computer Vision, vol. 72, no. 3, pp. 239-257 (2007), which is hereby incorporated by reference for this example. In real world most of the objects are not purely diffusive, and can be specular or hybrid. Recently new techniques have been explored to address objects that are not diffusive. Some methods have been proposed to address microgeometry measurement on specular surfaces. Examples of such methods can be found in Chen, T., et al, Mesostructure from specularity,” Proc. CVPR, pp. 1825-1832 (2006) and Francken, Y., et al., “High quality mesostructure acquisition using specularities,” in Proc. CVPR, p. 1-7 (2007), which are hereby incorporated by reference for these examples. These proposed methods either use hand-moved point light source or structured illumination to reconstruct depth from specularity, but their approaches can only capture images with limited number of lighting directions and therefore may result in insufficient samplings of the reflectance field. In another example, an object is pressed into an elastomer skin to remove the specular reflection of the object, and a photometric stereo technique is used to estimate the surface normal based on reflection from the covered skin. An example can be found in U.S. Patent Publication 20130033595, titled “HIGH-RESOLUTION SURFACE MEASUREMENT SYSTEMS AND METHODS,” by Adelson, H., and Johnson, Micah K., which is hereby incorporated by reference for this example. However, when using this approach, the true reflection from the sampled object is blocked by the elastomer skin, and information about the color or other reflectance properties is lost.
In the field of computer graphics, some research has been done on the simultaneous acquisition of surface geometry and reflectance properties for computer graphics rendering. For example, a photometric stereo technique for objects with spatially varying bidirectional reflectance distribution function (BRDF) can be found in Goldman, D. B., et al. “Shape and spatially-varying BRDFs from photometric stereo,” IEEE Trans. PAMI, vol. 32, no. 6, pp. 1060-1071 (2010), which is hereby incorporated by reference in its entirety. In this example, multiple images are captured under different lighting directions, and then the shape and reflectance parameters are reconstructed from a BRDF model for rendering. However, this method requires that the camera and illumination be at far distance from the sample in order to fulfill the orthographic assumption, and the method is unable to capture detailed microgeometry of the surface due to limited number of illumination directions. In addition, a reflectometry technique has been proposed for surface normal, height, diffuse and specular reflectance parameters measurement, but this method requires a linear light source be moved across the object surface. An example of such a reflectometry system can be found in U.S. Pat. No. 6,919,962, titled “Reflectometry apparatus and method,” which is hereby incorporated by reference for this example. A specular object scanner for measuring reflectance properties of objects has also been proposed, but this scanner requires an arc-shaped light source to be rotated around the object. An example of such a specular object scanner can be found in U.S. patent application Ser. No. 14/212,751 titled “Specular object scanner for measuring reflectance properties of objects,” which is hereby incorporated by reference for this example. In addition, a 12-light hemispherical dome for capturing detailed microgeometry of skin texture has been developed, which may improve the realism of facial synthesis. An example of such a system is found in Graham, P., et al., “Measurement-based synthesis of facial microgeometry,” in Computer Graphics Forum, vol. 32, no. 2, pt. 3. Wiley Online Library, pp. 335-344 (2013), which is hereby incorporated by reference for this example. However, in this example, the 12-light hemispherical dome has limited angular samplings above the surface, and may not work well for specular objects that are smoother. To acquire much denser samplings, the example discusses using a light stage, but the light stage is large and some of the lights may be occluded by the camera.
In these conventional techniques, the camera and multiple illuminations are placed at a far distance in order to fulfill the orthographic assumptions. Such a setup typically introduces a large form factor, and some of the illuminations may be occluded by the camera. Because the camera is not able to capture images of the surface closely, it is difficult to achieve micron-level depth resolution and accuracy. These conventional techniques use a limited number of extended light sources, and are typically arranged with large angular sampling steps due to the constraints in their physical dimensions. This may result in insufficient samplings of the reflectance field, and bias the surface normal estimation for specular surfaces that are relatively smoother. Moreover, typically some extended illuminations were placed at oblique angles introducing severe shadow effects. Most of these conventional techniques are unable to measure the reflectance properties of the surface.